Ventricular tachycardia (VT) and ventricular fibrillation (VF) are serious, life-threatening forms of cardiac arrhythmias. Implantable cardioverter defibrillators, or “ICDs”, are capable of automatically detecting arrhythmias and delivering anti-arrhythmia therapies. Delivering anti-tachycardia pacing therapies or high-energy shock therapies may terminate VT and VF. Ventricular tachycardia termination is typically referred to as “cardioversion.” Ventricular fibrillation termination is typically referred to as “defibrillation.”
Detection of an arrhythmia by an ICD is generally determined by comparing the sensed heart rate to predetermined, programmable parameters. The intervals between sensed events in the atria, referred to P-waves, and/or sensed events in the ventricles, referred to as R-waves, may be used to determine a heart rate. Generally, the interval between two sensed cardiac events, an R—R interval in the ventricle or a P—P interval in the atrium, is compared to a set of programmable detection intervals. For example a sensed R—R interval may be compared to a specified VT detection interval, a fast VT detection interval and a VF detection interval. If the sensed R—R interval is less then any one of these intervals, it is classified as such. Arrhythmia detection is made when a specified number of intervals in a detection interval range is reached. For example, a nominal setting for detecting VT may be 16 consecutively sensed intervals less than 400 ms. A nominal setting to detect VF may be 18 of the last 24 sensed intervals must be less than 320 ms. These settings may be adjusted according to patient need, however, once programmed by a clinician, generally remain fixed until the next programming session.
Nearly all of detected arrhythmias appropriately treated by an ICD do not result in death. However, ICD therapies can be very painful to the patient, and compromised hemodynamic output during a VT or VF episode can render a patient unconscious resulting in related serious injuries or death. Because of the serious consequences of cardiac arrhythmias, it is desirable to predict the occurrence of an arrhythmia so that preventive measures may be taken to avert the arrhythmia entirely.
Arrhythmia prevention therapies can include medical regimes, pacing regimes, or involve neurostimulation such as spinal cord stimulation. Reference is made to U.S. Pat. No. 6,134,470 issued to Hartlaub, incorporated herein by reference in its entirety. Continuous delivery of arrhythmia prevention therapies may not be practical due to side effects, cost or other factors. Reliable prediction of an imminent arrhythmia would allow preventative therapies to be delivered only when needed. An arrhythmia prediction must be made in ample enough time to allow a preventative therapy to be effective. The prediction time required will depend on the type of therapy to be delivered and may vary from on the order of a day, several hours, several minutes or several seconds.
A number of parameters for predicting a discrete VT or VF episode have been proposed including, for example, left ventricular dysfunction, myocardial ischemia, frequency of ventricular ectopic beats, heart rate variability, heart rate turbulence, or other electrocardiographic changes (see Shusterman et al., J Am Coll Cardiol. 1998;32:1891–9, and Schmidt et al., Lancet. 1999;353:1390–96). Changes in the autonomic nervous system are known contributing factors to arrhythmogenesis. The heart rate is normally regulated by a balance between the sympathetic and parasympathetic (vagal) components of the autonomic nervous system. Increased sympathetic activity, referred to as sympathetic tone, increases the heart rate and decreases heart rate variability. Increased vagal tone decreases the heart rate and increases heart rate variability. Heart rate variability (HRV) is the variation in consecutive heart rate cycles. Changes in autonomic tone, especially in conjunction with myocardial ischemia can play an important role in the development of arrhythmias. Therefore, indicators of changes in autonomic tone may be useful in predicting arrhythmias. Reference is made to U.S. Pat. No. 5,042,497 issued to Shapland, U.S. Pat. No. 5,318,592 issued to Schaldach, and U.S. Pat. No. 5,658,318 issued to Stroetmann et al.
Other methods for predicting arrhythmias based on changes in a sensed cardiac electrograms (EGM) or a patient's ECG are generally disclosed in U.S. Pat. No. 6,115,627 issued to Street, U.S. Pat. No. 6,308,094 issued to Shusterman, U.S. Pat. No. 4,458,691 issued to Netravali, and U.S. Pat. No. 5,271,393 issued to Callaghan. In U.S. Pat. No. 6,272,377 issued to Sweeney et al., an estimated arrhythmia probability is calculated based on detected conditioning events statistically associated with an arrhythmia.
Patients may also experience non-sustained arrhythmias, which terminate spontaneously without any medical intervention. Arrhythmia detection algorithms used by ICDs typically discriminate between a non-sustained arrhythmia and a sustained arrhythmia based only on static detection parameters regarding the duration of the arrhythmia. Arrhythmia detection is generally absolute in that either a detection is made, followed by an associated treatment, or no detection is made and no treatment is delivered. The difference between a sustained arrhythmia requiring treatment and a non-sustained arrhythmia that spontaneously terminates is generally determined by fixed arrhythmia detection parameters programmed by a physician. For example, if the number of intervals required to detect an arrhythmia is programmed to 16, an arrhythmia that lasts 15 intervals long and spontaneously terminates will not be detected at all, while an arrhythmia that lasts at least one interval longer will be detected and may be treated.
The inventors of the present invention hypothesize that the underlying factors that may trigger a sustained arrhythmia may be the same factors that trigger a non-sustained arrhythmia. A sustained arrhythmia may represent a worsening condition of these factors, which, in a less severe state, trigger arrhythmias that spontaneously terminate. A worsening condition, it is hypothesized, may first present itself as an increase in the frequency or duration of non-sustained arrhythmias and ultimately in a sustained arrhythmia. Other changes in the characteristics of non-sustained arrhythmias, such as EGM changes related to the cycle length and signal morphology, may move toward that typical during a sustained arrhythmia.
In a retrospective study of ICD patients performed by the inventors, patients having episodes of non-sustained VT were much more likely to experience a sustained VT or VF episode than patients that did not experience non-sustained VT episodes. Furthermore, the number of non-sustained episodes and the total number of non-sustained arrhythmia cycles per day increased dramatically on the day that a sustained VT or VF occurred. The atrial interval (PP interval) and ventricular interval (RR interval) during non-sustained VT became more similar to the PP and RR intervals measured during a sustained VT. Thus, the inventors of the present invention hypothesize that trends in the incidence of non-sustained arrhythmias may be useful predictors of the occurrence of a sustained arrhythmia. It is desirable therefore, to provide a method for monitoring the incidence of non-sustained arrhythmias and for determining a metric of non-sustained arrhythmias within an individual patient. Such a metric may be used for predicting the occurrence of a sustained arrhythmia.
Such a metric may also be useful in adjusting arrhythmia detection parameters to improve the ability of the ICD to dynamically discriminate between non-sustained and sustained arrhythmia episodes. Patients that experience frequent non-sustained arrhythmia episodes may be exposed to repeated anti-arrhythmia therapies if recurring non-sustained arrhythmias are detected by the ICD according to static detection parameters, before the episode has time to self-terminate. Anti-arrhythmia therapies can be painful to the patient, consume ICD battery energy, and, in some cases, accelerate or otherwise worsen the severity of the arrhythmia.
By dynamically adjusting arrhythmia detection parameters based on a metric of non-sustained arrhythmias, the delivery of unneeded anti-arrhythmia therapies may be reduced, conserving battery energy needed for terminating sustained arrhythmias and sparing the patient potentially painful therapies. Moreover, the metric of non-sustained arrhythmias may be used for the successful prediction of a sustained arrhythmia that would allow arrhythmia prevention therapies to successfully avert the need for anti-arrhythmia therapies, prevent other injuries that can occur with an arrhythmia, and, most importantly, prevent potentially fatal arrhythmias from occurring.